Polymer(Korea), Vol. 38, No. 6, pp. 726-734
http://dx.doi.org/10.7317/pk.2014.38.6.726
ISSN 0379-153X(Print)
ISSN 2234-8077(Online)
유리섬유강화 에폭시 레진 복합체의 기계적, 유전체 특성에 미치는
첨가제 함유 에폭시 영향
Cuong Manh Vu†, Liem Thanh Nguyen*, Thai Viet Nguyen, 최형진**,†
Chemical Deparment, Le Qui Don Technical University,
*Polymer Center, Ha Noi University of Science and Technology, ** 인하대학교 고분자공학과
(2014년 3월 22일 접수, 2014년 4월 27일 수정, 2014년 4월 28일 채택)
Effect of Additive-added Epoxy on Mechanical and Dielectric Characteristics of
Glass Fiber Reinforced Epoxy Composites
Cuong Manh Vu†, Liem Thanh Nguyen*, Thai Viet Nguyen, and Hyoung Jin Choi**,†
Chemical Deparment, Le Qui Don Technical University, No. 236 Hoang Quoc Viet, Ha Noi, Viet Nam
*Polymer Center, Ha Noi University of Science and Technology, No.1, Dai Co Viet, Ha Noi, Viet Nam
**Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea
(Received March 22, 2014; Revised April 27, 2014; Accepted April 28, 2014)
Abstract: Three different types of additives, thiokol, epoxidized natural rubber (ENR) and epoxidized linseed oil (ELO),
were dispersed in an epoxy matrix before being used in glass fiber (GF) composites, and their effects on the mechanical
and dielectric properties of epoxy resin and glass fiber reinforced epoxy composites (GF/EP) were examined. The addition of each of 7 phr ENR, 9 phr ELO and 5 phr thiokol into the epoxy resin increased the fracture toughness significantly
by 56.9, 43.1, and 80.0%, respectively, compared to the unmodified resin. The mode I interlaminar fracture toughness
of the GF/EP at propagation was also improved by 26.9, 18.3 and 32.7% when each of 7 phr ENR, 9 phr ELO, and 5 phr
thiokol, respectively, was dispersed in the epoxy matrix. Scanning electron microscopy showed that the additives reduced
crack growth in the GF/EP, whereas their dielectric measurements showed that all these additives had no additional effect
on the real permittivity and loss factor of the GF/EP.
Keywords: glass fiber, epoxy resin, fracture toughness, dielectric property.
Introduction
modes of GF/EP could be controlled gradually. Epoxies are
used widely as a polymer matrix for high performance laminated composites because of their good mechanical performance, processability, and compatibility to most fibers with
chemical resistance, wear resistance and low cost. These materials, however, are relatively brittle after curing, which is detrimental to the interlaminar properties. Therefore, substantial
efforts have been made towards improving the fracture toughness by toughening the matrix material. Their mechanical performance would be enhanced dramatically by incorporating a
small amount of nano-materials, such as carbon nanotube,
nano-clay, nano-cellulose, and nano-thermoplastic fibers or
micro-fillers to the epoxy matrix,2 whereas it has been reported
that epoxy with smaller size fillers has better fracture performance than that with larger-size fillers.3-5
The toughening effect of nano-filler on the GF/EP was also
examined.6-10 Recently, Liu et al.11 carried out a systematic
Fiber reinforced composites are used widely in the fields of
space, aircraft, automobile and sports industries because of
their high strength and stiffness to weight ratios. On the other
hand, it is essential to examine the resistance against delamination propagation, which is useful in the design and analysis
of composite structures because one of the limitations of these
materials is their poor ability to resist impact and delamination.1
The strength properties of glass fiber reinforced epoxy composites (GF/EP) are sensitive to the failure modes, such as
transverse cracking, longitudinal cracking and delamination
between the adjacent layers, due to the intrinsic brittleness of
the epoxy matrix. Recent studies showed that most failure
†
To whom correspondence should be addressed.
E-mail: [email protected]; [email protected]
726
Effect of Additive-added Epoxy on Mechanical and Dielectric Characteristics of GF/EP
study on the toughening of bulk epoxy using nano-silica, nanorubber and nano-silica/nano-rubber particles. Compared to the
nano-silica particles, nano-rubber was found to have a more
significant effect on the toughness. The Mode I fracture toughness of the rubber-modified epoxy increased with increasing
particle loading up to 15 wt%, but the Young’s modulus of the
composite reduced to 78% of the neat epoxy. The fracture
toughness of the hybrid nano-silica and nano-rubber particles
in the epoxy had no synergistic effect other than the sum of the
toughness value due to each nano-particle.11
Liquid rubber, such as carboxyl-terminated butadiene acrylonitrile (CTBN) rubber, has potential as a modifier for GF/EP
without changing the viscosity.12,13 When this system is cured,
the epoxy polymerizes and CTBN reacts with the epoxy to
form a copolymer. With increasing molecular weight, the soluble reactive liquid CTBN rubber phase separates from the
epoxy because of the decrease in rubber/epoxy compatibility.
The elastomeric phase forms small discrete particles, typically
in the micrometer range, which are dispersed in and bonded to
the epoxy matrix. Although the morphology of rubber in
toughened epoxy systems is mostly spherical, the mean particle size and distribution can vary considerably, depending on
the curing reactions, cure cycle and concentration of rubber in
the epoxy system.14-17 Abadyan et al.18 examined the rubber
modification of hoop filament wound epoxy composites using
amine-terminated butadiene acrylonitrile (ATBN) and CTBN
oligomers for two different hybrid modified epoxies. In one
system, the epoxy was modified by ATBN and hollow glass
spheres as fine and coarse modifiers, respectively. The other
hybrid epoxy was modified by a combination of ATBN and
recycled tire particles. The measured fracture toughness of the
blends revealed synergistic toughening for both hybrid systems
in some formulations.19
The fracture toughness of resins modified with liquid rubber
increased with increasing CTBN content up to 17%. The maximum fracture toughness of the modified epoxy was approximately 30 times that of the unmodified epoxy, whereas the
fracture toughness of vinylester resin modified with CTBN
was approximately four times that without modification. The
CTBN modifier also increased the ductility by almost 100% at
the expense of concomitant reductions in strength and modulus. Despite the large increase in fracture toughness of the
bulk resin, the same increase was not guaranteed when the
modified resin was used as an adhesive or as a matrix material
for fiber-reinforced composites up to 200% improvements in
interlaminar fracture toughness was reported for carbon fiber-
727
CTBN-modified epoxy matrix composites.20
On the other hand, to design unmanned aerial vehicles, there
is a demand for radio frequency transparency. Therefore, it is
essential to understand both mechanical properties and dielectric properties of GF/EP. The dielectric properties of GF/EP in
the centimeter wave range (8.2-12.4 GHz) were measured
using the free-space method reported by Seo et al..21
This study examined the production and characterization of
epoxidized natural rubber (ENR), epoxidized linseed oil
(ELO), and thiokol-modified glass fiber/epoxy laminated composites. An epoxy system already optimized for toughness
through the incorporation of conventional modifiers was chosen as the baseline material. The mechanical properties of the
modified epoxy resin containing different ENR, ELO, and
thiokol contents were compared with the unmodified epoxy
resin. Based on these results, GF/EP made of 7 phr ENR, 9 phr
ELO, and 5 phr thiokol modified epoxy resin were manufactured to determine the mechanical and dielectric properties,
including tensile strength, Mode I interlaminar fracture toughness, and dielectric constant.
Experimental
Materials. The epoxy resin (EP) used in all experiments was
diglycidal ether of bisphenol A (DER 331, Dow Chemical
Co.), and dietylentriamine (DETA) (Dow Chemical Co.) was
used as a curing agent. The ENR of Figure 1 (epoxide percentage: 50%, Mn=10000-20000 g/mol), supplied by the Vietnamese Military Institute of Science and Technology, was
prepared by reacting the natural rubber latex (NR) of cis-1,4isoprene with performic acid (product of reaction between formic acid and hydrogen peroxide), as shown in Figure 2. The
ELO (viscous liquid, light yellow color, epoxide percentage:
21.6%, iodine value: 2, 4 g of I2/g, acid value: 0.5 mg KOH/
g, density at 20 oC: 1-1.2 g/cm3 and boiling point is higher than
200 oC) was purchased from Akcros Chemicals (UK). Thiokol
was Thioplast G21 from Akazo Nobel. The woven roving Eglass fiber with area weight of 300 g/m2 (WRE300) was purchased from Jiujiang Beihai Fiberglass Co., China.
Preparation of Samples. Solutions of each additive of
ENR (5-20 phr), ELO (5-20 phr) and thiokol (3-9 phr) with
different concentrations in the epoxy resin were first mixed
together using a mechanical stirrer, and heated for 1 h at
around 60 oC in a water bath to ensure proper dispersion of the
additive. The mixtures were cooled to room temperature, while
a curing agent was added prior to hand mixing for approxPolymer(Korea), Vol. 38, No. 6, 2014
728
Cuong Manh Vu, Liem Thanh Nguyen, Thai Viet Nguyen, and Hyoung Jin Choi
imately 20 min. The products were degassed using a vacuum
pump. Next, the resin mixture was poured into a 4 mm-thick
mould that was coated with a release agent. The samples were
pre-cured at room temperature for 24 h and then post-cured at
80 oC for 3 h. At this stage the cured specimens were allowed
to cool slowly at room temperature. Once all formulations
were made, they were then reinforced with hand layup glass
fiber process to form composite sheets. They were finally
cured at the same temperature cycle which was used for resin
specimens. The glass fiber volume fraction was 50±2%.
Resin Fracture Toughness Test. The single edge notch
bend (SENB) specimens (Figure 3) according to the ASTM
(D5045-99) were used to test the fracture toughness (critical
stress intensity factor, KIC). The notch tip was machined using
a rotating saw, and the pre-crack of specimens was then generated by tapping on a fresh razor blade placed in the notch.
The fracture toughness tests were conducted at a cross-head
speed of 10 mm/min. The KIC value reported represents the
average of at least five tests. The following eq. (1) was used to
calculate the KIC:
⎛ P ⎞
Q ⎟
KIC = ⎜⎜ ---------- f( x )
1⎟
--⎝
2⎠
BW
(1)
With
1
2
--2 [ 1.99 – x ( 1 – x ) ( 2.15 – 3.93x + 2.7x ) ]
f( x) = 6x --------------------------------------------------------------------------------------3
( 1 + 2x ) ( 1 – x )
Figure 1. Structures of compounds.
Figure 2. Epoxidation reaction of natural rubber.
폴리머, 제38권 제6호, 2014년
--2
where PQ is a critical load for crack propagation (kN), B is the
specimen thickness (cm), W is the specimen width (cm), f (x)
is the non-dimensional shape factor, a is the crack length (cm),
and x=a/W.
Tensile Test. The tensile test was performed using the
Instron 5582-100KN machine according to the ISO-527-1993.
The specimen gauge length was 50±1 mm and the testing
speed was set to 2 mm/min. The specimen dimension was
250×25×2.5 mm. Glass fiber reinforced plastic/epoxy tabs
with a thickness of 1.5 mm were attached at both ends of the
specimen by an adhesive. The values were taken from a mean
of five specimens.
Mode-I Interlaminar Fracture Toughness Test. The
mode I double cantilever beam (DCB) tests were carried out
using the ASTM (D5528-01). The recommended specimen
size is at least 150 mm long and 20 mm wide with an initial
crack length (i.e. the length of the insert from the line) of
50 mm. Hinges of the same width as the specimen were
attached to allow the application of a load. The load and dis-
Figure 3. Schematic diagram of the KIC specimen.
Effect of Additive-added Epoxy on Mechanical and Dielectric Characteristics of GF/EP
Figure 4. Geometry of DCB specimen (all dimensions in mm).
placement were then related to the delamination length as measured with a ruler on the specimen edge (Figure 4). The modeI interlaminar fracture toughness GIC and GIP for each additive contents was calculated using the modified beam theory
(MBT) method as follows:
3Pcδ F
GIC = ----------------------- ---2b( a + ∆ ) N
3PPδ F
GIP = ----------------------- ---2b( a + ∆ ) N
729
Figure 5. Photograph of the free space measurement system (PNA
8362B -Agilent USA).
(2)
space reflection (S11) and transmission (S21) coefficients of
the planar samples were measured after fixing the sample sheet
at the reference plane. A mixture of the dielectric can also be
found from the S11 and S21 parameter measurements. Complex permittivity, ε* = ε' − jε'', for each additive contents was
calculated with respect to the frequency22 with dielectric permittivity (ε') and loss factor (ε'').
(3)
Results and Discussion
where GIC is the fracture toughness at an initial crack stage
corresponding to the first peak load in the force-displacement
curves, GIP is the fracture toughness at the propagation stage
that is taken from the plateau region of the R-curves, PP is the
applied load, C is the compliance corresponding to each crack
length, a is the crack length, Pc is the initial maximum load, b
is the specimen width, δ is a load point deflection, and ∆ is an
effective delamination extension to correct rotation of the DCB
arms at the delamination front. In addition, N is the end-block
correction factor, and F is a large displacement correction factor.
Morphology Analysis. The morphology was examined by
both scanning electron microscopy (SEM) (Joel JSM 6360,
Japan) and field emission scanning electron microscopy (FESEM) (S-4800, Hitachi), in which the fractured samples under
mechanical analysis were sputter-coated with gold prior to the
test.
Dielectric Measurement. The dielectric characteristics
were examined using the basic free space measurement system
consisting of a network analyzer (an Agilent PNA network
analyzer), a sample holder, and two horn antennas with Agilent
8362B software. Two horn antennas were used as a transmitter
and receiver, respectively, and the samples were placed
between these two, as shown in Figure 5. The measurements
were carried out in the frequency range of 4-8 GHz. The free-
Resin Fracture Toughness. Figure 6 shows the results of
resin fracture toughness for neat epoxy, 5-20 phr ENR, 5-20
phr ELO, and 3-9 phr thiokol. The KIC values of epoxy resin
improved significantly by adding each ENR, ELO, and
thiokol. The addition of ENR, ELO, and thiokol imparts an
increase in the KIC value up to an optimal content of 7, 9 and
5 phr, respectively. At 7 phr ENR, the fracture toughness
improved by 56.9% from 0.65 to 1.02 MPa.m1/2. At 9 phr
ELO, the fracture toughness increased by 43.1% from 0.65 to
0.93 MPa.m1/2, whereas at 5 phr thiokol, the fracture toughness
increased by 80% from 0.65 to 1.17 MPa.m1/2. No further
increase was observed upon further loading of the additives.
This may be attributed to the bigger size of rubber particles at
higher concentrations. As noted from SEM pictures there is an
increase in the size of particles for 15 phr ENR, 15 phr ELO
Figure 6. Effect of ENR, ELO, and thiokol contents on KIC of the
epoxy resin.
Polymer(Korea), Vol. 38, No. 6, 2014
730
Cuong Manh Vu, Liem Thanh Nguyen, Thai Viet Nguyen, and Hyoung Jin Choi
Figure 7. (a) ENR; (b) ELO aggregation in 15 phr blends.
blends (Figure 7). The large particles obtained at high additive
content would deteriorate impact toughness as this would
result in high stress intensity around the agglomerated rubber
particles. The improvement in toughness attributed to additive
particles that enhance the shear localization by acting as stress
concentrators, crack deflections and crack pinning processes at
obstacles, which was suggested to play an important role in
toughening.15,23 To better understand the toughening mechanism of the additives in the epoxy resin, the fractured surfaces
of the specimens at zone ahead of the crack tip were observed
by SEM. Figure 8(a) and (b) show that the fractured surface of
the neat epoxy was smooth and glassy, which was the typical
brittle fracture behavior of a thermosetting polymer. A smooth
mirror-like surface with micro-flow lines was observed.
On the other hand, the fractured surface of the modified
epoxy at 7 phr ENR, 9 phr ELO and 5 phr thiokol was rougher,
and jagged multi-plane patterns appeared so that more energy
was required. SEM of the ENR (Figure 8(c), (d)), ELO (Figure
8(e), (f)), and thiokol (Figure 8(g), (h))-modified systems
revealed the presence of additive particles (ENR, ELO, and
thiokol), which were dispersed throughout the epoxy matrix,
i.e. they showed the presence of a two-phase morphological
feature. The soft elastomeric phase in the case of ENR and
thiokol was phase-separated from the hard epoxy matrix during the early stages of curing. The fractured surfaces of the
most of the elastomer-toughened epoxy systems had a rigid
폴리머, 제38권 제6호, 2014년
Figure 8. SEM images of the fractured surfaces of (a and b) neat
epoxy, (e and f) 9 phr ELO, (c, d) 7 phr ENR, (g, h) 5 phr thiokol.
The direction of crack propagation is from top to bottom.
continuous epoxy matrix with a dispersed rubbery phase as
isolated particles. Different mechanisms, such as crazing, shear
bonding and elastic deformation of the rubber particles have
been proposed, and these mechanisms were believed to act
alone or in the rubber particles, and these mechanisms were
thought to act alone or in rubber-modified epoxy.15,24
Tensile Strength. The effect of additive contents on the
large strain of the GF/EP was investigated up to their failure.
Table 1 shows the tensile strength and Young’s modulus for
Table 1. Tensile Strength and Young’s Modulus for GF/EP at
Different Contents of Additives
Tensile strength
(MPa)
Young’s modulus
(GPa)
Unmodified
187.94
6.792
7 phr ENR
185.60
6.708
9 phr ELO
182.34
6.625
5 phr thiokol
194.28
6.802
Effect of Additive-added Epoxy on Mechanical and Dielectric Characteristics of GF/EP
731
Figure 9. SEM images of the fractured surfaces of the GF/EP for
the tensile testing specimens: (a) unmodified; (b) 5 phr thiokol; (c)
7 phr ENR; (d) 9 phr ELO.
GF/EP at different additive contents. For the modified composite, the tensile strength decreased slightly with the addition
of 7 phr ENR and 9 phr ELO. These can be attributed to the
fact that the modulus of ENR and ELO is much lower than that
of the epoxy matrix as well as glass fiber. In addition, low
modulus ENR and ELO particles act as stress concentrators
and decrease the yield strength.25 Although there is a tendency
to increase tensile strength for 5 phr thiokol modified GF/EP,
the increase was no significant.
The Young’s modulus, determined from the typical stressstrain curve, showed little decrease for composites modified
with 7 phr ENR, and 9 phr ELO. The Young’s modulus with
addition of 5 phr thiokol slightly increased.
SEM clearly showed the broken fiber pull-out at the fractured surface of the glass fiber as well as the fiber breakage for
both the unmodified composite and modified composite, as
shown in Figure 9(a), (b), (c) and (d). The most distinct feature
of the fractured surface with the addition of elastomers of
thiokol and ENR was the extensive matrix deformation
between the clean fiber and matrix. Although there was extensive matrix deformation, the tensile strength of the GF/EP was
not strongly affected by the addition of the additives. The addition of additives which are characterized by large surface areas
per unit gram plays an important role in determining the
strength of the interface. The addition of additives leading to
the number of adhesively jointed points with glass fibers was
very low, which had no effect on tensile strength of the GF/EP.
Mode-I Interlaminar Fracture Toughness Test. The
delamination resistance curves (R-curves) are drawn between
the crack length (a) and the corresponding fracture toughness,
as shown in Figure 10. The GIC value corresponding to the
Figure 10. Typical delamination resistance (R-curves) of the GF/EP
with various additive contents: (a) unmodified; (b) 5 phr thiokol; (c)
7 phr ENR; (d) 9 phr ELO.
first crack initiation was determined from the load point at
which the initiation of delamination is observed at the microscopic level on the specimen edge. The crack growth rate of
the unmodified GF/EP propagates smoothly in wide steps as a
result of the relatively low tenacity of the polymeric phase with
glass fibers. On the other hand, the crack of the GF/EP with
5 phr thiokol, 7 phr ENR and 9 phr ELO was deflected and
pinned gradually by the reinforcing obstacles so that more
energy was required, resulting in higher fracture toughness.
The delamination initiation (GIC) values were reported
throughout this investigation corresponding to first peak load
in the load-crack opening displacement curves, whereas the
delamination propagation (GIP) values were taken from the
plateau region of the R-curves.26
The mode-I interlaminar fracture toughness of the GF/EP
was determined using the DCB test, in which the curves of the
Polymer(Korea), Vol. 38, No. 6, 2014
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Cuong Manh Vu, Liem Thanh Nguyen, Thai Viet Nguyen, and Hyoung Jin Choi
Figure 11. SEM images of the fractured surface of GF/EP for the
fracture toughness testing specimen: (a) unmodified; (b) 5 phr
thiokol; (c) 7 phr ENR; (d) 9 phr ELO.
Figure 12. Typical force-displacement curves of the (a) unmodified;
(b) 5 phr thiokol; (c) 7 phr ENR; (d) 9 phr ELO.
applied load vs. displacement were recorded. Figure 11
showed that the force increased linearly until the maximum
force value was reached, then decreased gradually in the manner of a zigzag shape (stick-slip) in the propagation stages.
For the modified composite, both the displacement and force
values in Figure 12 were higher than those of the unmodified
composite. The crack was suggested to propagate more stably
and gradually as a result of the relatively high tenaciousness of
the epoxy modified with the elastomer. Moreover, the mode-I
interlaminar fracture toughness values were calculated using
the MBT method, as shown in Figure 13.
A significantly increased in the mode-I interlaminar fracture
toughness with the modified epoxy resin additives was
observed. At 5 phr thiokol, 7 phr ENR and 9 phr ELO content,
the crack initiation (GIC) increased by 30.3, 26.9 and 17.6%,
respectively, and the GIP values also increased by 32.7, 26.9
and 18.3%, respectively, compared to the unmodified composite. The presence of thiokol, ENR, and ELO in GF/EP
acted as an obstacle reinforcement that deflected, pinned and
폴리머, 제38권 제6호, 2014년
Figure 13. Initiation (GIC) and propagation (GIP) fracture toughness in mode I of the GF/EP composite with various contents of
additives.
delayed the crack propagations, so more energy was required.
The major energy absorption mechanism in the composite,
which are: crack deflection, debonding between the fiber and
resin, pull-out (extraction of the fibers from the resin), and
fiber-bridging mechanism.13,27 In general, a number of mechanisms contribute to the fracture toughness, and it is often very
difficult to determine the dominant mechanism. SEM of the
fractured surfaces of specimens revealed clear damage in the
interfacial region in the composite. For the unmodified composite (Figure 11(a)), the fractured surfaces were mostly
smooth and glassy due to brittle failure. Therefore, the energy
required for the interlaminar delamination failure was low. In
contrast, the additives strongly affected the mode-I interlaminar fracture toughness, as shown in Figure 11(b), (c), and (d).
The fractured surface of the additive-modified composite had
a rougher surface and was tougher than those of the unmodified composite. Therefore, more energy was required, resulting in higher fracture toughness.
The dielectric property, which is a measure of the polarizability of a material when subjected to an electric field, is an
important factor in defining the physical and chemical properties related to the storage and energy loss in various materials. The permittivity (ε), which is also known as a material’s
dielectric constant, describes the interaction of a material with
an electric field. The dielectric constant is equivalent to the relative permittivity or the absolute permittivity (ε) relative to the
permittivity of free space. The real part of the permittivity (ε')
is a measure of how much energy from an external electric
field is stored in a material. The imaginary part of the permittivity (ε'') is called the loss factor and is a measure of how
Effect of Additive-added Epoxy on Mechanical and Dielectric Characteristics of GF/EP
733
ified and unmodified GF/EP were 0.0239, 0.0243, 0.0235, and
0.024, respectively, suggesting little external electric field loss
and confirming that thiokol, ENR, and ELO had no added
effect on the dielectric properties of the GF/EP.
Conclusions
Figure 14. Variations of the real permittivity (ε') and loss factor (ε'')
as a function of frequencys for GF/EP with unmodified (purple
line), 5 phr thiokol (red line), 9 phr ELO (blue line), and 7 phr ENR
(black line).
Table 2. Dielectric Properties of the Modified and Unmodified
GF/EP
Materials
Unmodified
Dielectric properties
ε'
ε''
3.178
0.0240
5 phr thiokol
3.457
0.0239
7 phr ENR
3.791
0.0243
9 phr ELO
3.625
0.0235
the dissipative or loss of a material is to an external electric
field.
Figure 14 shows the measured permittivity of the specimens,
in which the ENR, ELO, and thiokol-modified GF/EP have a
similar real permittivity and loss factor line compared to the
unmodified GF/EP, indicating that the thiokol, ENR, and ELOmodified GF/EP had no effect on the external electric field
compared to the unmodified GF/EP. The average results of the
dielectric properties measurements were calculated from
4 GHz until 8 GHz, and are presented in Table 2. The average
real part of the permittivity (ε') of the unmodified GF/EP, and
GF/EP modified with 5 phr thiokol, 7 phr ENR, 9 phr ELO
were 3.178, 3.457, 3.791, and 3.625. The average real part of
the permittivity (ε') with the addition of thiokol, ENR, and
ELO increased slightly. The small values of the real part of the
permittivity showed that there was no more external electric
field stored in the material, which agrees with the loss factor.
The loss factor of 5 phr thiokol, 7 phr ENR, 9 phr ELO mod-
By introducing a novel GF/EP modified with thiokol, ENR,
and ELO, we examined the effects of additives on the mechanical and dielectric properties of GF/EP in this study. With the
addition of 5 phr thiokol, 7 phr ENR, and 9 phr ELO, the fracture toughness of the epoxy resin increased by 80.0, 56.9 and
43.1%, respectively. The mode-I interlaminar fracture toughness of GF/EP containing 5 phr thiokol, 7 phr ENR and 9 phr
ELO contents for the GIC increased by 30.3, 26.9 and 17.6%,
respectively, and for the crack propagation, the GIP values also
increased by 32.7, 26.9 and 18.3%, respectively, compared to
the unmodified composite. The dielectric properties obtained
from the free space method for frequencys 4-8 GHz at room
temperature of 28 oC showed that all these additives had no
added effect on the dielectric properties of the GF/EP.
The GF/EP modified with 5 phr thiokol showed the best
mechanical properties compared to the 7 phr ENR and 9 phr
ELO modified GF/EP.
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